The present invention relates to a method of forming patterned photoresist features that are useful for fabricating thin film circuits, such as magnetic, electronic, and semiconductor circuits.
In the fabrication of electronic and magnetic circuit components, photolithographic processes are used to form miniaturized devices having features that are often sized less than 1 .mu.m. In conventional photolithographic methods, a layer of light-sensitive resist material is applied on a substrate, and a photomask is used to expose the resist to a desired pattern of radiation, such as ultraviolet light. The photomask can be superimposed on the resist (contact method), or a photomask can be projected onto the resist (projection method). Thereafter, the exposed photoresist is developed to form a pattern of raised photoresist features 10 on the substrate 12, as illustrated in FIG. 1.
In modern circuit fabrication, proper control of the shape of the photoresist features 10 formed on the substrate is highly desirable. For example, when subsequent processing steps deposit material on, or etch material from, between the resist features 10, it is desirable for the resist features to have sidewalls 14 that are substantially vertical and perpendicular to the surface of the substrate. However, conventional photolithographic processes often produce resist features 10 having a spread-out foot 16 at the bottom of the features, as schematically illustrated in FIG. 1, that result from the chemical interaction of the resist layer with the substrate surface 18.
Conventional methods have been developed to reduce the spreading out of the foot 16 of the resist feature 10. For example, Dean and Carpio, in OCG Interface 94 Proceedings, "Contamination of Positive Deep-UV Photoresist," Sematech, Austin, Tex. (1994), teach that a coating of silicon dioxide on the substrate can serve as a barrier which prevents undesirable chemical reaction between the resist and the substrate, allowing the resist features to have a substantially vertical sidewalls, and to minimize spreading-out at the bottom of the resist feature. However, such coatings are often difficult to remove on completion of the resist utilizing fabrication step, without adversely affecting the underlying or adjacent materials on the substrate surface. Also, the additional step adds to processing cost and reduces processing throughput.
Moreover, in certain fabrication processes, it is desirable for the resist features to have cross-sectional profiles that are reentrant at the bottom of the features. By "reentrant profile," it is meant that the sidewalls of the resist features taper inwardly at the bottom of the feature, and more preferably form an elongated trench parallel to the substrate surface, the trench positioned along the junction of the resist feature and the substrate surface. Such a profile is particularly desirable when material is conformally deposited on and between the resist features to form electrical interconnect lines or to fill holes on the substrate. In these processes, a solvent is used to remove the residual resist, in a process commonly known as a lift-off process. The material conformally deposited over the resist features (which is often highly chemically resistant) prevents penetration of the solvent below the conductive layer to allow removal of the resist. However, the reentrant resist features cause a shadowing effect during deposition, that results in formation of an elongated void substantially absent deposition material, at the edge of the bottom surface of the resist feature in contact with the substrate. It is through this void that the solvent enters and dissolves the residual resist.
Conventional methods used to obtain reentrant resist features using negative or image-reversal photoresist underexpose the bottom portion of the resist layer by limiting the exposure of the resist layer to radiation. However, these processes are sensitive to the chemical species on the surface of the substrate which affects the chemical reactions between the overlying resist and the substrate. Because different substrates have different surface chemistry and activity, such processes have often limited applications and reproducibility. Other methods have been used to control the profile of features formed using positive photoresist material. These methods typically chemically modify the surface of the resist layer to decrease the solubility of the surface of the resist to provide a larger width at the top surface as compared to the bottom of the feature. In one method, the resist layer is immersed into chlorobenezene solution; in another method, vapor silanation is used. However, many complex and difficult-to-control variables affect the surface chemistry of the features. Also, these methods often result in poor adhesion between the resist and the underlying substrate, and can also provide inconsistent resist development times. In yet another method, multiple coatings, typically 2 to 3 coatings, are used to produce the reentrant profiles. These methods depend on differences in the etch rate of the layers in a developing agent or plasma to produce the reentrant profiles. However, these processes use multiple and often complex process steps that can increase the cost of processing the substrate.
Thus it is desirable to have a process for forming resist features having controlled and predefined cross-sectional profiles. It is further desirable to have a process for forming resist features having reentrant profiles at the bottom of the features. It is also desirable for the resist forming process to use materials that are chemically compatible with the substrate to reduce corrosion and/or contamination of the substrate during formation of the resist features. It is further desirable for the process to be compatible with conventional processes to allow easy integration into conventional manufacturing apparatus.